A permanent magnet starter/generator (PMSG) or integral starter/generator (ISG), collectively referred to herein as a permanent magnet starter/generator, or PMSG, is driven in the generator mode by a gas turbine shaft at speeds ranging from zero rotations per minute to 50,000 rotations per minute or more to generate electricity for the engine and accessories.
These variations in driving speeds result in proportional variations in PMSG output voltage. The output voltage of the PMSG also varies as various loads are applied, this effect is also referred to as a V-I load characteristic, an example of which is shown in FIG. 1. For instance, at aircraft take-off, when turbine speed is highest, a maximum voltage of 110 Volts AC may be generated when the generator is not loaded, while in an engine idle mode, a 30 Volts AC voltage may be generated by the PMSG under full load conditions. A sample V-I load characteristic for 55V is shown in FIG. 1.
Aircraft onboard equipment is usually driven using 28 volts DC voltage. Furthermore, a variable load current may be drawn by the engine and aircraft accessories. Therefore, the wild voltage and frequency AC electricity generated by the PMSG is preferably regulated into a constant voltage DC output to drive these accessories. Although some regulation solutions are known, there remains a need for a scheme which provides regulation across the complete spectrum of speeds and loads present in the typical prime-mover gas turbine PMSG system.
One technique, known as saturation control, employs saturable core reactors in conjunction with a pulse width modulation scheme, typically a switched-mode power supply or SMPS, wherein the output current flowing through the saturable inductor saturates the inductor at high currents, and does not saturate the inductor at lower currents, and thereby may regulate output voltage.
This device is sometimes referred to as a magnetic amplifier or “Magamp”. Saturation-based voltage regulation systems, however, are typically not effective at all load current levels, and therefore improvement is needed.
Furthermore, it will be appreciated that in the case of aircraft applications, it is necessary to provide the output voltage with a very high reliability, at low cost and using a low weight solution. The technological challenges are therefore many.
There is therefore a need for a voltage regulation method and apparatus that will address at least the above-identified challenges.